1. Field of the Invention
This invention relates to a method and apparatus for the remote display of percentage modulation data obtained at a transmitter site, and more particularly to a method and apparatus which will accomplish the accurate transmission and display of percentage modulation data obtained from a modulation monitor at a transmitter site and transmitted by conductive means to the remote site.
The Federal Communications Commission (FCC) requires the continuous monitoring of the modulation of all broadcast stations including amplitude modulated (AM), frequency modulated (FM) and television (TV). The equipment used to measure percentage modulation is called a modulation monitor. Its accuracy and other characteristics are specified in the Rules and Regulation of the FCC. Modulators are type approved by the FCC at their laboratory in Laurel, Maryland. Broadcasters are required to have a modulation monitor at the transmitter location and, if they operate with remote control, a second monitor at the remote control position.
Proper operation of the monitor at the remote control point is often very difficult to achieve. This is because the "off the air" carrier sample is not sufficiently pure for accurate measurements. This can be particularly true for the most critical measurement of 100% negative modulation of AM carriers. Even small local noise sources may contaminate the carrier sample to preclude accurate measurements. Of course, there is also the disadvantage of the expense of the second monitor.
2. Discussion of the Prior Art
Patents representative of the known prior art are:
U.S. Pat. No. 2,074,737 PA1 U.S. Pat. No. 2,078,285 PA1 U.S. Pat. No. 2,496,259 PA1 U.S. Pat. No. 2,779,868 PA1 U.S. Pat. No. 3,343,030 PA1 U.S. Pat. No. 3,345,625 PA1 U.S. Pat. No. 3,641,554.
U.S. Pat. Nos. 2,074,737 and 2,078,285 both broadly disclose the use of cathode ray tubes for visual display of modulation information, however, the systems disclosed are quite different and non-suggestive of the invention disclosed herein. U.S. Pat. No. 2,078,285 relates to a system for determining the character and percentage of modulation of a radio transmitter at a point remote from the transmitter. Unlike the system of the present invention which measures percentage modulation at the transmitter and produces a digital representation of percentage modulation which is transmitted conductively to a remote site where the digital value is used in creating a bar graph pulse representing the digital value which is displayed on a TV display, the system disclosed in U.S. Pat. No. 2,078,285 derives modulation information from a radio signal received at a station remote from the transmitter. The system disclosed in U.S. Pat. No. 2,078,285 is therefore subject to inaccuracies resulting from contamination of the carrier sample during transmission which the present invention is designed to avoid.
U.S. Pat. No. 2,779,868 discloses a radio remote control system having a meter for indicating percent modulation, however, no description of the operation of the percent modulation meter appears in the specification.
U.S. Pat. Nos. 3,343,030 and 3,345,625 broadly disclose apparatus for producing bar graph displays, and U.S. Pat. No. 3,641,554 discloses a system which converts analog data from a number of sources to digital form, stores the digital data in a memory having a plurality of channels, selectively retrieves digital data from any one of the channels and converts it back to analog form for graphic display as a trace on a cathode ray tube.
The method and apparatus of the invention disclosed herein is not suggested by any of the prior art patents listed above.